Not Applicable
Carbon monoxide can be produced by reforming hydrocarbon feed with steam and carbon dioxide at high temperatures. The reaction can occur in a steam methane reformer (SMR) which contains catalyst-filled tubes housed in a furnace. The synthesis gas (also called syngas) exiting the reformer contains carbon monoxide along with hydrogen, carbon dioxide, steam and unconverted methane according to the equilibrium established from the following reactions:
The synthesis gas is cooled and separated into various products, i.e., carbon monoxide or syngas of different hydrogen to carbon monoxide (H2/CO) ratios, recycle carbon dioxide, and by-product hydrogen, using separation technologies such as amine absorption, adsorption and/or cryogenic separation.
Carbon monoxide, the principal product of a reformer plant, is used in the manufacture of isocyanates and polycarbonates through phosgene chemistry. Other processes, such as certain oxoalcohol production processes, require a low 1:1 H2/CO ratio synthesis gas. In SMR, with a natural gas feed and with full carbon dioxide recycle from the process gas only, the lowest H2/CO ratio obtainable is about 3. Thus, hydrogen is typically produced far in excess of what is required. Some hydrogen can be used to fire the reformer; the remainder must be exported at fuel value. The requirement of finding a fuel home for this excess hydrogen introduces a constraint. Also, using a value-added material such as hydrogen to substitute for readily available fuels is not the most economical choice.
As is well known in the industry, injecting carbon dioxide into the reformer hydrocarbon feed very effectively reduces the H2/CO ratio towards what is required. Besides recycling carbon dioxide separated from the synthesis gas product, additional carbon dioxide can be obtained from the furnace flue gas or imported into the plant from an outside source. Under conditions of complete CO2 recycle, the reformer steam to carbon (S/C) ratio is irrelevant in the determination of the H2/CO ratio. However, it determines the amount of carbon dioxide recycled from the process gas. The combination of recycled carbon dioxide and imported carbon dioxide yields the reformer carbon dioxide to carbon (CO2/C) ratio. For a given H2/CO ratio there is an infinite combination of the reformer S/C and CO2/C ratios. Table 1 illustrates this for a particular case in which H2/CO=0.6, pressure=200 psia (1379 kPa), and temperature=1750xc2x0 F. (955xc2x0 C.).
As the S/C is lowered, a lower CO2/C ratio is required to produce the specified syngas H2/CO ratio. With lower S/C and CO2/C, process steam requirement and carbon dioxide recycle are reduced, and methane leakage increases. Producing steam and recycling carbon dioxide are expensive in energy, capital, and power. Hence it is desirable to operate the reformer at the lowest permissible S/C ratio, consistent with a tolerable methane leakage. However, low S/C operation increases the potential of depositing carbon on the reforming catalyst. Carbon deposition deactivates the catalyst resulting in high reactor tubewall temperature and reduced hydrocarbon conversion. It can also block the reformer tubes resulting in high pressure drops inside the reactor. Several carbon formation mechanisms can be involved:
Conventional steam methane reformer conditions are limited to the point of incipient carbon deposition predicted by the carbon forming reaction equilibria. At every temperature between the reformer inlet and outlet, an equilibrated gas composition can be calculated for the given feed stoichiometry. The reaction equilibrium constant for any of the carbon forming reactions can be calculated (K-operating), and compared with the critical value for depositing carbon via that reaction mechanism (K-equilibrium). The K-operating/K-equilibrium ratio should exceed 1 everywhere in the reformer. The minimum value of this ratio as computed for the Boudouard reaction is shown in Table 1. The table shows that in the production of synthesis gas having H2/CO ratio of 0.6, the minimum desirable S/C is about 2.31, which gives a narrow operating margin away from conditions where carbon forms. S/C ratios of 1 and 2 are unacceptable because the operating Boudouard K-value is less than the equilibrium value for this reaction and the Boudouard reaction will proceed to the right hand side of the chemical equation as written above, resulting in carbon deposition. The necessity of operating above an S/C of 2.3 implies a CO2/C greater than 4.5. The resulting huge carbon dioxide recycle would be accompanied by unacceptably high costs.
It is thus highly desirable to develop a process that produces syngas with a H2/CO ratio of 1 or lower and reduces the S/C ratio, yet avoids carbon formation. Such a process would have the following advantages:
reduced costs associated with the recovery and recompression of carbon dioxide;
reduced costs associated with steam generation;
reduced flows, sizes, and duties of all heat exchangers, including the firebox; and
greater energy efficiency in operation of the plant.
U.S. Pat. No. 4,782,096 (Banquy, 1988) and U.S. Pat. No. 4,888,130 (Banquy, 1989) disclose processes for producing a synthesis gas having a H2/CO ratio below 2.5. In the processes a hydrocarbon feed is split into two parts. A portion is reformed in a primary reformer at high temperature and pressure. The effluent is combined with the remainder of feed and undergoes a secondary reforming reaction in an adiabatic reactor by reacting with an oxygen rich stream. The overall S/C is very low; e.g., 0.4 in one example. Carbon deposition is reportedly avoided by maintaining the combined feed to the second (autothermal) reformer above a minimum temperature (e.g., 1100xc2x0 F. or 594xc2x0 C.), and by designing burners with high mixing efficiency. However, the process of U.S. Pat. No. 4,888,130, as shown in the examples, achieves syngas H2/CO ratios greater than 1.5. Also, the process needs expensive oxygen feedstock.
Syngas H2/CO ratios of less than 2 can be achieved using autothermal reforming. Carbon formation is avoided by using well mixed burners. With full recycle of carbon dioxide, the minimum syngas H2/CO achievable is 1.6. This ratio can be lowered further with imported carbon dioxide. However, autothermal reactors require pure oxygen, and unless inexpensive oxygen is available, they are not economical in comparison with fired reformers. In addition, the oxygen consumption goes up with the amount of carbon dioxide or water recycled or imported into the feed, as necessitated by lower and lower H2/CO ratios. Air cannot be used as an oxidant source because the products will be contaminated with nitrogen which is difficult to remove from a hydrogen-carbon monoxide syngas or carbon monoxide product. Also, elaborate safety and shutdown systems are required to handle hot oxygen in the presence of fuel.
U.S. Pat. No. 2,199,475 (Wilcox, 1940) discloses the addition of a controlled amount of carbon dioxide to hydrocarbon gases and steam prior to introduction into the dissociation chamber of a reactor. Oxygen can be added, preferably at an intermediate point in the dissociation chamber. The oxygen is fully consumed in partial combustion of unreformed methane (and hydrogen and carbon monoxide). Temperatures in excess of 2000xc2x0 F. are thereby reached where the reforming and reverse shifting occur without a catalyst. The resulting equilibrium favors methane extinction and reverse shift towards lower H2/CO ratios. The example in the patent produces a syngas H2/CO ratio of 2. The same penalties as discussed above with autothermal reformer apply here, with regard to oxygen consumption and oxygen safety. Additionally, the process requires two separate reactors.
U.S. Pat. No. 3,723,344 (Reynolds, 1973) and U.S. Pat. No. 3,919,113 (Reynolds, 1975) disclose a process for producing oxo-synthesis gas by partial oxidation of a hydrocarbon fuel with oxygen, splitting the effluent into two parts, shifting one part to a higher H2/CO ratio, and simultaneously reverse shifting the other part to a lower H2/CO ratio. Carbon is formed, and scrubbed off. This is feasible and practical since the entire process is non-catalytic. The forward-shifted syngas is stripped of carbon dioxide which is compressed to feed the reverse shift reaction. Thus the process maintains an overall high H2/CO ratio. The process uses at least 3 reactors, plus carbon removal systems. In addition, there are oxygen consumption and safety issues.
The oxygen penalty is circumvented by the process disclosed in U.S. Pat. No. 1,903,845 (Wilcox, 1933) in which the following steps are carried out in a refractory filled bed: burning fuel in air in order to heat the bed; purging the bed with methane; and introducing methane, carbon dioxide and steam at one end of the hot bed where reforming occurs and cools the bed. The major drawback of this process is the requirement for two parallel trains for quasi-continuous operation.
U.S. Pat. No. 3,103,423 (Pearce, 1963) and GB 2,015,027 A (1979) disclose that injecting small quantities (2 ppm) of hydrogen sulfide into a hydrocarbon feed can lead to carbon-free reforming, under conditions that otherwise cause severe carbon deposition. The sulfur passivates the reforming catalyst allowing for low steam to carbon feed ratios without carbon formation problems. This concept has been commercialized in what is referred to in the industry as the SPARG process.
U.S. Pat. No. 3,859,230 (Moe, 1975) discloses a process for production of synthesis gas in which a naphtha and steam mixture is passed through a first reforming zone to produce an effluent containing methane, steam, hydrogen, and carbon dioxide. The effluent from the first reforming zone is divided in a major and a minor portion. Carbon dioxide is separated from the minor portion and then combined with the major portion prior to passing the major portion through a second reforming zone. The process is reported to increase the carbon monoxide content relative to hydrogen in the synthesis gas product.
DE 2,711,991 A1 (1978) discloses a process for producing synthesis gas with high carbon monoxide content by reacting a mixture of hydrocarbons and carbon dioxide in a tubular furnace reformer. Carbon dioxide feed is obtained, in part, by separating it from the furnace flue gases.
DE 3,501,459 C2 (1991) discloses a process for producing a H2/CO synthesis gas and a pure CO fraction. In the process, hydrocarbons are converted into reformed gas in the presence of carbon dioxide as the oxygen supplying component. Carbon dioxide is recycled after removal from the reformed gas.
GB 2,170,508 A (1986) discloses a method for producing a stoichiometric hydrogen/carbon monoxide gas by catalytically converting a hydrocarbon stream, in the presence of carbon dioxide, by endothermic catalytic oxidation. It is reported that a syngas having H2/CO ratios from about 0.3 to 2.3 can be obtained by using feedstocks having CO2/C ratios of about 0.3 to 5 and H2O/C ratios of about 0 to 5.
According to the present invention, hydrogen/carbon monoxide synthesis gas (also called syngas) is produced by injection of a second reactant stream into a hydrocarbon steam reformer at a location between the entry and discharge ends of the reformer. The second reactant stream can comprise one of the following reactants:
carbon dioxide;
a mixture of carbon dioxide and hydrocarbon;
a mixture of hydrocarbon and steam,
a mixture of carbon dioxide and steam; or
a mixture of carbon dioxide with hydrocarbon and steam,
wherein all or part of the mixtures containing hydrocarbon and steam can be prereformed hydrocarbon in steam.
In the process of this invention a first reactant stream comprises a hydrocarbon feed, typically natural gas, that is heated, desulfurized, and mixed with steam so that the ratio of steam to carbon (S/C) ranges from 1 to less than 3. The hydrocarbon/steam mixture is then further heated, optionally prereformed and reheated, and passed through a reformer.
The second reactant stream is heated and injected into the reformer at an intermediate point between the entry and discharge ends of the reformer. The quantity of reactant in the second reactant stream is controlled to produce a reformer effluent with a ratio of hydrogen to carbon monoxide (H2/CO) ranging from about 0.6 to less than 3.
Hydrocarbons in the first and second reactant streams are prereformed, if necessary, to reduce the hydrocarbons heavier than methane.
One embodiment of the present invention comprises the steps of:
heating and mixing a desulfurized hydrocarbon feed with steam;
prereforming the hydrocarbon/steam feed, if necessary, to reduce the amount of hydrocarbons heavier than methane;
introducing the heated hydrocarbon/steam mixture into catalyst tubes contained in a reformer heated to operating temperature;
introducing a second reactant stream, as described above, into the reformer at a location between an entry end and a discharge end; and
adjusting the quantity of reactant in the second reactant stream to produce a reactor effluent with a ratio of H2/CO ranging from about 0.6 to less than 3.
According to the present invention, when the second reactant stream is a mixture of steam and hydrocarbon, the S/C ratio in the injected mixture is lower than the S/C ratio in the first reactant stream.
In another embodiment of the present invention, the S/C ratio in the mixed hydrocarbon and steam feed can be about 1.4. It is heated to about 1000xc2x0 F. (538xc2x0 C.) before entering the reformer. Carbon dioxide, the second reactant stream, is heated and injected into the reformer at a temperature of 1450xc2x0 F. (788xc2x0 C.) to 1550xc2x0 F. (844xc2x0 C). A product syngas having H2/CO ratio ranging from 0.6 to 1 can be achieved.
Intermediate injection of a second reactant stream provides the following advantages:
a syngas effluent having a low H2/CO ratio can be produced; and
substantially no carbon deposition occurs during the reforming process.